In many of the conventionally-known vehicular steering apparatus, a road-wheel turning mechanism is connected to a steering wheel, and tire or road wheels are steered by steering force of the steering wheel via the road-wheel turning mechanism. In recent years, so-called steer-by-wire (“SBW”) type vehicular steering apparatus have been under development, in which the road-wheel turning mechanism is mechanically separated from the steering wheel, and in which a road-wheel turning actuator generates road-wheel turning power in response to a steering operation amount and the road wheels are steered by the thus-generated road-wheel turning power being transmitted to the road-wheel turning mechanism. Examples of such steer-by-wire type vehicular steering apparatus are disclosed in Japanese Patent Application Laid-Open Publication No. 2008-189077 (hereinafter referred to as “patent literature 1”) and Japanese Patent Application Laid-Open Publication No. 2008-195187 (hereinafter referred to as “patent literature 2”).
In the vehicular steering apparatus disclosed in patent literature 1 and patent literature 2, the road-wheel turning mechanism is connected to the steering wheel via two connection paths, i.e. electric and mechanical connection paths. The mechanical connection path is used as a backup for the electric connection path. Normally, the steering wheel and the road-wheel turning mechanism are interconnected only via the electric connection path. Once the electric connection path is canceled or deactivated for some reason, the connection between the steering wheel and the road-wheel turning mechanism is automatically switched to the mechanical connection path.
In the normal operating state, where the steering wheel and the road-wheel turning mechanism are interconnected only via the electric connection path, the road wheels can be steered under optimal conditions on the basis of composite information comprising a combination of steering information (such as a steering angle and speed) input by a human driver and external information, such as a vehicle velocity, added to the steering information. Thus, in the normal operating state, a high maneuverability of the vehicle can be achieved. In the normal operating state, the steering wheel and the road-wheel turning mechanism are mechanically separated from each other as noted above, so that there is no restriction or limit to the steering angle of the steering wheel. Thus, some limit has to be put on the steering angle of the steering wheel for the following reason.
Today, many of the steering wheels are equipped with an airbag device rotatable with the steering wheel, and such an airbag device and a control section fixed to the vehicle body for issuing control signals to the airbag device are interconnected via a cable that is wound on a cable reel attached to the steering wheel. The number of times the cable reel can rotate is limited by the number of turns of the cable on the reel. Thus, if there is no limit on the steering angle (number of rotations) of the steering wheel, an excessive load would be imposed on the cable reel and cable.
Because the steering angle of the steering wheel ordinarily exceeds 360°, it is not possible to put a limit on the steering angle by means of a mere stopper alone. Therefore, a mechanism for putting a limit on the steering angle of the steering wheel tends to be extremely complicated in construction. One conceivable approach for putting a limit on the steering angle of the steering wheel is to use a steering reaction motor provided, for example, in the above-mentioned electric connection path. The steering reaction motor generates steering reaction force against steering force with which the steering wheel is operated by the driver, i.e. generates steering operation resistance to be applied to the steering wheel in a rotating direction of the steering wheel. Namely, the above-mentioned conceivable approach is intended to cause the steering reaction motor to generate an extremely great steering reaction force when the steering angle of the steering wheel has reached a predetermined rotational angle.
However, the above-mentioned conceivable approach requires a high-power steering reaction motor that tends to unavoidably become large in size and weight, which would lead to an increased size and cost of the steering apparatus. For this reason, the above-mentioned conceivable approach is not so a good approach.